III-Nitride High Electron Mobility Transistor (HEMT) devices are often used in power applications and/or high temperature applications in RF circuits and in other applications, including in power supplies for electrically powered motor vehicles.
A design trade-off between the on-state resistance (Ron) and breakdown voltage (BV) of a HEMT can be improved significantly by following the teachings contained herein. Since the relation between the BV and Ron is at least quadratic, improvement in the BV for a given drift
region length results in a significant improvement in the figure of merit (FOM) of the device, defined as BV2/Ron.
HEMTs utilize two semiconductor materials with different band-gaps, forming an electron potential well at a heterointerface between the two semiconductor materials, which materials might be, for example, AlGaN and GaN. The potential well confines electrons and defines a two-dimensional electron gas (2DEG) conduction channel. Due to the two-dimensional nature of the electrons in the conduction channel, the carrier mobility is enhanced.
Prior art III-Nitride HEMTs utilize a uniform 2DEG density which results in a peak electric field under or near the gate region. The electric field distribution tends to be closer to a triangular shape than to a more desirable trapezoidal shape which reduces the BV per unit drift region length of the device. The use of a field plate and/or multi step field plates are some of the techniques that are used in the prior art to improve the electric field distribution but these techniques typically result in multiple peaks and suffer from less than ideal flat field distribution (they can exhibit a saw tooth type profile) which also adds to the gate to drain capacitance. In addition, process complexity and cost typically increase with the number of field plate steps (levels) utilized.
The prior art includes:
Furukawa, U.S. Pat. No. 7,038,253 issued on May 2, 2006 discloses a GaN based device that represents state of the art GaN on Si technology which uses a uniform 2DEG profile in the drift region. In the absence of any field shaping technique it is expected that the breakdown and dynamic Rdson performance of the device of this patent will be limited by a localized increase in the electric field under the gate region thus requiring over design of the device which degrades the Figure of Merit (FOM) that can be achieved by such a structure.
H. Xing et al. have proposed a device structure that was published in a paper entitled “High Breakdown Voltage AlGaN/GaN HEMTs achieved by Multiple Field Plates”, (see H. Xing, Y. Dora, A. Chini, S. Hikman, S. Keller and U. K. Mishra, “High Breakdown Voltage AlGaN—GaN HEMTs Achieved by Multiple Field Plates,” IEEE Electron Device Letters, IEEE ELECTRON DEVICE LETTERS, VOL. 25, NO. 4, pp. 161-163, April 2004), which utilizes a field shaping technique that used multiple field plates to improve the electric field distribution, however, this technique is less favorable than the technology disclosed herein since multiple field plates will not achieve a uniform electric field (will have a saw tooth type distribution) and will increase the gate to drain capacitance. In implementing such structure increases device complexity and cost.
C. M. Waits, R. Ghodssi, and M. Dubey, “Gray-Scale Lithography for MEMS Applications”, University of Maryland, Department of Electrical and Computer Engineering, Institute for Advanced Computer Studies, College Park, Md., USA, 2006.
W. Henke, W. Hoppe, H. J. Quenzer, P. Staudt-Fischbach and B. Wagner, “Simulation and experimental study of gray-tone lithography for the fabrication of arbitrarily shaped surfaces,” Proc. IEEE Micro Electro Mechanical Syst. MEMS 1994, Oiso, Japan, pp. 205-210.
C. M. Waits, R. Ghodssi, M. H. Ervin, M. Dubey, “MEMS-based Gray-Scale Lithography,” International Semiconductor Device Research Symposium (ISDRS), Dec. 5-7, 2001, Washington D.C.
In another aspect the present invention relates to a HEMT device comprising: a substrate; a buffer layer disposed above said substrate; a carrier supplying layer disposed above said buffer layer; a gate element penetrating said carrier supplying layer; a drain element disposed on said carrier supplying layer; wherein the carrier supplying layer has a non-uniform thickness between said gate element and said drain element, the carrier supplying layer having a relatively greater thickness adjacent the drain element and a relatively thinner thickness adjacent the gate element.
In yet another aspect the present invention relates to a HEMT device having a non-uniform two-dimensional electron gas conduction channel formed in a carrier supplying layer of the HEMT device between a gate element of the HEMT device and a drain element of the HEMT device, the HMET device also having a constant electric field distribution between said gate element and said drain element.
In still yet another aspect the present invention relates to a HEMT device comprising: a substrate; a buffer layer disposed above said substrate; a carrier supplying layer disposed above said buffer layer; a gate; a drain disposed on said carrier supplying layer; wherein a two dimensional electron gas (2DEG) is formed between the gate and the drain and wherein the carrier supply layer is configured to adapt the 2DEG such that a variation in electric field strength as a function of a distance between the gate and the drain is substantially constant.